One layer of membrane decides whether your dormer stays dry

A dormer is a small structure with a lot of exterior: two cheeks, a front face and a little flat roof, all around a single room. So there is relatively much insulation in little space — and where there is insulation, the subject is vapour.

The rule is short. Dutch consumer authority Milieu Centraal advises ensuring a vapour-retarding layer on the warm side, that is, beneath the insulation. Warm indoor air carries moisture; if that air reaches the insulation package unhindered, it cools there and the moisture condenses at the coldest point. The vapour layer stops that air before it gets the chance.

The mistake that follows is as simple as it is persistent: adding another layer. Milieu Centraal warns that wood trapped between two vapour-retarding layers can become too damp and start to rot. In a renovation where someone adds an extra membrane "to be safe", or where new insulation goes over old without knowing what sits underneath, exactly that sandwich appears — and the wood caught inside it is the load-bearing structure of your dormer.

In practice: during a renovation, always have it established what is already in the construction before anything is added. A contractor unwilling to open it up is guessing. What a dormer renovation costs (€ 1,500-6,000) sits alongside the replacement prices in the price index — and that gap is exactly why keeping the structure sound pays.